You should ensure that the default values for the initial fluid volume fraction are appropriate for your simulation. The default volume fraction for dispersed fluids is zero, and for continuous fluids it is 1/N, where N is the number of continuous phases. Note that you can leave one fluid to use the Automatic option - the remaining volume fraction will be assigned to this fluid such that the volume fractions sum to unity. For cases involving free-surface flows, there are additional considerations for setting the initial conditions. For details, see Initial Conditions.